As will be appreciated by those skilled in the art, there are various reasons for choosing a Universal Serial Bus (USB) port to supply charging power to a mobile device, rather than using a separate alternating current (AC) charger. Unfortunately, USB ports can only provide limited power.
Power rails of mobile devices often require a very clean supply voltage. Thus, Low Drop-Out (linear mode) circuits have been used to meet this requirement. A problem with these circuits is that they can be quite inefficient power converters.